Describe why oxygen moves from the air to the blood in the alveolar capillaries and again why oxygen moves from the blood to the interstitial fluid in the body tissues. why is the partial pressure (concentration) of oxygen so low in the interstitial fluid?

Higher partial pressure of oxygen in alveolar air than that of alveolar capillaries drives the diffusion of oxygen gas from air to the blood.

Higher partial pressure of oxygen in blood at tissue level as compared to the interstitial fluid drives diffusion of oxygen from blood to the tissue fluid.

Consumption of oxygen for cellular respiration in cells reduces the its concentration in the interstitial fluid.

Explanation:

Gaseous exchange in the body occurs through the process of diffusion wherein respiratory gases are exchanges down the concentration gradient.

Since the alveolar air has a higher partial pressure of oxygen than that of blood present in the surrounding blood capillaries, oxygen diffuses from the alveolar air to the blood making the blood oxygen-rich.

As the oxygen-rich blood reaches the body tissues, oxygen is diffused from the blood into the interstitial fluid since the later has lower oxygen concentration.

The supply of oxygen from the blood to the body tissues is required as the cells consume the oxygen to perform aerobic respiration and retrieve energy from the nutrients.

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.